No. 596] 



EVOLUTIONARY THEORY 



4:>9 



ing up of one of the genes that makes for the normal foot 

 might have a residuum that positively interferes with the 

 operation of other foot-forming factors. 



The genetical studies also accord with other evidence 

 as to the general irreversibility of evolution. We see 

 that when a population contains only a recessive trait it 

 is impossible by breeding inside that population to get 

 back the dominant allelomorph. From a pure blue-eyed 

 population we do not get back the primitive brown-eyed 

 condition. 



It might be thought that if evolution proceeds chiefly 

 by loss of factors it would tend to simplification rather 

 than increase of the number of factors. It seems prob- 

 able, however, that the loss is not merely of a whole gene, 

 but of some part of it; a fractionation, as it were, by 

 which the gene becomes altered or split up into two or 

 more. For example, it is probable that in man the loss 

 of a gene (or a part of one) releases special nervous 

 states, as, for instance, that in which musical combina- 

 tions run through the brain, or numerical relations are 

 rapidly worked through, etc. Thus the general result of 

 experimental work in genetics gives support to the view 

 of evolution by loss of genes and by their fractionation. 



4. Evidence from Evolutionary Changes in the Inor- 

 ganic World. Radiation Studies.— The view that evolu- 

 tion is primarily by internal changes receives unexpected 

 support from the recent discoveries concerning the evolu- 

 tion of the elements. It is now well known that the ele- 

 ment, uranium, under certain conditions of temperature, 

 etc., undergoes a spontaneous change into ionium, ionium 

 into radium, radium into polonium, an essentially lead- 

 like substance. Similarly, thorium passes through 

 mesothorium and radiothorium. Indeed, in a particular 

 series, side branches may be given off; thus polonium is 

 not derived directly from radium, or from radium C, but 

 from one form of radium C called radium C,. Indeed, 

 "radiums" of different sorts, called radium A, radium 

 B, radium C, are recognized and the atoms of these differ 



